JP3747242B2 - Optical pulse repetition frequency multiplier - Google Patents

Optical pulse repetition frequency multiplier Download PDF

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JP3747242B2
JP3747242B2 JP2002070036A JP2002070036A JP3747242B2 JP 3747242 B2 JP3747242 B2 JP 3747242B2 JP 2002070036 A JP2002070036 A JP 2002070036A JP 2002070036 A JP2002070036 A JP 2002070036A JP 3747242 B2 JP3747242 B2 JP 3747242B2
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optical
frequency
repetition frequency
optical pulse
cavity
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JP2003270691A (en
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カント グプタ カマル
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National Institute of Information and Communications Technology
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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/42—Coupling light guides with opto-electronic elements
    • G02B6/4201—Packages, e.g. shape, construction, internal or external details
    • G02B6/4204—Packages, e.g. shape, construction, internal or external details the coupling comprising intermediate optical elements, e.g. lenses, holograms
    • G02B6/4215—Packages, e.g. shape, construction, internal or external details the coupling comprising intermediate optical elements, e.g. lenses, holograms the intermediate optical elements being wavelength selective optical elements, e.g. variable wavelength optical modules or wavelength lockers
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
    • G02B26/002—Optical devices or arrangements for the control of light using movable or deformable optical elements the movement or the deformation controlling the frequency of light, e.g. by Doppler effect
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06791—Fibre ring lasers
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29346—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by wave or beam interference
    • G02B6/2935—Mach-Zehnder configuration, i.e. comprising separate splitting and combining means
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29346—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by wave or beam interference
    • G02B6/29361—Interference filters, e.g. multilayer coatings, thin film filters, dichroic splitters or mirrors based on multilayers, WDM filters
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/21—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  by interference
    • G02F1/212—Mach-Zehnder type
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F2203/00—Function characteristic
    • G02F2203/54—Optical pulse train (comb) synthesizer
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/081—Construction or shape of optical resonators or components thereof comprising three or more reflectors
    • H01S3/082—Construction or shape of optical resonators or components thereof comprising three or more reflectors defining a plurality of resonators, e.g. for mode selection or suppression
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
    • H01S3/1062—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using a controlled passive interferometer, e.g. a Fabry-Perot etalon
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
    • H01S3/1106—Mode locking
    • H01S3/1109—Active mode locking
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
    • H01S3/1106—Mode locking
    • H01S3/1121—Harmonically mode locking lasers, e.g. modulation frequency equals multiple integers or a fraction of the resonator roundtrip time

Description

【０００１】
【発明の属する技術分野】
本発明は，光パルス繰り返し周波数逓倍装置に関するものであり，特に，アクティブ・ハーモニカリ・モードロック・ファイバリングレーザ（ＭＬ−ＦＲＬ）に関するものである。アクティブ・ハーモニカリ・モードロック・ファイバリングレーザ（ＭＬ−ＦＲＬ）において生成された光パルス列の繰り返し周波数を逓倍するとともに光パルスを安定化させるものである。
【０００２】
本発明は，高速度光通信システム，光時分割多重システム（ＯＴＤＭ）のための光パルス発生源に使用できるようにしたものであり，そのために必要とされる高い繰り返し周波数の光パルス列を安定化するものである。あるいは，本発明はマイクロ波周波数帯，ミリ波周波数帯，およびそれ以上の周波数帯での繰り返し周波数の光パルス列を安定化するものである。
【０００３】
【従来の技術】
アクティブ・ハーモニカリ・モードロック・ファイバリングレーザ（ＭＬ−ＦＲＬ）は高い繰り返し周波数の光パルスを生成することができるために，高速度光通信におけるＯＴＤＭ／ＷＤＭのための非常に一般的なパルス源になってきた（参考文献１，２等）。ここに，キャビティ共振周波数の高調波のＲＦ変調信号は，キャビティ内部におかれたマッハ・ツェンダ強度変調器（ＭＺＭ）のＲＦポートに印加され，そしてその伝送特性曲線のクワドゥラチュールポイントにバイアスされる。それにより，印加された変調信号の周波数に等しい繰り返し周波数の光パルスを生成する。しかし，そのようなレーザ装置においては，最大パルス繰り返し周波数は，ドライブするエレクトロニクス機器の周波数特性および変調器の周波数バンド幅等により制限される。そのような高速度光通信システムのためには，もとになる光パルスの繰り返し周波数が高いことが重要である。ＭＬ−ＦＲＬにおいてパルス繰り返し周波数を増大するために様々な方法が提案されて，バンド幅の大きい変調器を用意する，あるいは高い周波数を発生できるエレクトロニクス機器を用意する等の要求を緩和した。例えば，キャビティ内部にマッハ・ツェンダ強度変調器（ＭＺＭ）を置くことによる周波数逓倍があり，変調器の非線形周波数特性がパルス繰り返し周波数の増大に利用されている（参考文献３，４）。あるいは，キャビティ内部にファイバ・ファブリ−ペロフィルタ（ＦＦＰフィルタ）を置くことによる光フィルタリングする方法があり，ＦＦＰフィルタにより縦振動モードを選択的にフィルタリングすることでＭＬ−ＦＲＬのパルス繰り返し周波数を増大できる（参考文献５，６等）。他の方法として，ＭＬ−ＦＲＬにおいてｐｆm の繰り返し周波数の光パルス列を連続発振動作で生成するために変調周波数を±ｆc ／ｐだけデチューニングすることにより高い繰り返し周波数の光パルス列を生成できる（参考文献７，８，９等）。
【０００４】
図１１（ａ）は従来のアクティブ・ハーモニカリ・モードロック・ファイバリングレーザ（ＭＬ−ＦＲＬ）の構成を示す。図１１（ａ）において，Ａは光増幅器であり，励起光源と光利得性能をもつ利得媒体により構成したものである。１は利得媒体であり，エルビウムドープファイバ（ＥＤＦ）であって，Ｅｒ／Ｙｂドープファイバである。２は励起光源である。３はカップラ（光結合器）である。４は変調器であって，マッハ・ツェンダ変調器である。５，６，７は光アイソレータである。８はポーラライゼーションコントローラ（ＰＣ）である。９は光結合器であって，９０：１０の割合で光ファイバリングの発振光を光検出器２３に分岐するものある（光ファイバリングの発振光の１０％が光検出器２３を通過する）。１２は光ファイバである。２１は電気発振器であって，高周波電気信号を発生するものである。２２は電気増幅器である。２３は光検出器であって，光信号を電気信号に変換するものである。２４は測定器であって，電気信号に変換された信号を測定するものである。
【０００５】
図１１（ａ）において，励起光源２で発生したレーザ光は利得媒体１であるエルビウムドープファイバ（ＥＤＦ）を励起し，光ファイバリングにおいてその共振周波数ｆc およびその整数倍のレーザ光（スーパーハーモニックモード）が発振する。電気発振器２１は，ｆc の整数倍の周波数ｆm の電気信号を発生し，変調器（マッハ・ツェンタ光強度変調器）４に印加する。変調器４には電圧Ｖb のバイアス電圧を印加する。
【０００６】
図１１（ａ）の構成で，変調器４は，伝送特性曲線のクワドゥラチールボント（伝送特性曲線における最大伝送点）に対応する電圧Ｖb にバイアスされ，印加変調周波数ｆm に等しい繰り返し周波数の光パルス列を光ファイバリングに生成する。図１１（ｂ）はそのように生成されて出力された繰り返し周波数ｆm の光パルス列を示す。
【０００７】
ＭＬ−ＦＲＬにおいてスーパーモードノイズを抑制する手段として複合キャビティを使用することはすでに知られている（参考文献１０）。また，前述したように，ＭＬ−ＦＲＬにおいてパルス繰り返し周波数を増大する方法としてＫ×ｆm に等しい自由空間周波数領域（ＦＳＲ）をもつＦＦＰフィルタをＭＬ−ＦＲＬキャビティに挿入し，ＭＬ−ＦＲＬのキャビティにおいて光フィルタリングすることにより，ＦＳＲの周波数間隔で支配される共振モードをエンハンスメントする方法がある（Ｋは整数，ｆm は印加変調周波数）。この方法により，ＭＬ−ＦＲＬによりＦＳＲに等しい繰り返し周波数での光パルス列を連続発振動作で生成することができる。しかし，このＦＦＰフィルタを使用する方法では，最大パルス繰り返し周波数は内部キャビティＦＦＰフィルタのＦＳＲに制限される。
【０００８】
【発明が解決しようとする課題】
上記のように，従来のＭＬ−ＦＲＬでは生成される光パルスの繰り返し周波数がｆm の整数倍に限られていた。また，生成される光パルスも振幅が不安定である等の問題があった。
【０００９】
本発明は，従来のＭＬ−ＦＲＬで得られる光パルスの繰り返し周波数を簡単な構成で逓倍でき，しかも光パルスの振幅等を安定化させることのできる光パルス繰り返し周波数逓倍装置を提供することを目的とする。
【００１０】
【課題を解決するための手段】
本発明は，アクティブ・ハーモニカリ・モードロック・ファイバリングレーザにおいて，印加変調周波数ｆm の整数倍の自由空間周波数領域（ＦＳＲ＝Ｋｆm ）をもつＦＦＰフィルタと，キャビティの長さの異なる複数のキャビティをもつ複合キャビティをメインキャビティに備えるようにした。ＦＦＰフィルタによるキャビティ内部の光フィルタリングと複合キャビティの複数キャビティのそれぞれの共振周波数の高調波のうちの共通の高調波をエンハンスメントすることにより繰り返し周波数の増大を図るようにした。本発明によれば，ＦＦＰフィルタと複合キャビティの組合せにより，キャビティの長さを調整する簡単な操作により，パルス繰り返し周波数を逓倍させることができる。
【００１１】
本発明の原理についてさらに詳述する。ＦＦＰフィルタのＦＳＲより高いパルス繰り返し周波数にさらに増大するために，ＦＳＲより高い繰り返し周波数をもつ高次キャビティ共振モードの振動をエンハンスメントすることが大切である。これを達成するために，本発明は，内部光フィルタリングと複合キャビティによりパルス繰り返し周波数を高くするようにした（図２参照）。ここに，複合キャビティは異なる長さの複数の光ファイバ（パッチコード）と５０／５０光結合器により構成される。キャビティの長さは複合キャビティの長さＬ1 とＬ2 をもつ一組のパッチコードＰ1 とＰ2 により調整される。Ｌ1 とＬ2 （Ｌ1 ＞Ｌ2 ）の長さは，長いキャビティと短いキャビティのキャビティ共振周波数ｆ’c1とｆ’c2がＦＳＲの倍数の高調波に等しくなるように，その最も低い共通周波数で干渉するように選ばれる。その結果，内部ＦＦＰを組み込んだ複合キャビティＭＬ−ＦＲＬは，Ｐ×ＦＳＲもしくはＰ×ｆm の間隔の周波数の間隔の支配的なキャビティ共振モードをエンハンスメントする。これは，複合キャビティＭＬ−ＦＲＬがｆ’composite ＝Ｐ×Ｋ×ｆm のような仮想的共振周波数ｆ’composite をもつことを表すものである。アクティブ・ハーモニック・モードロック動作のもとで，変調周波数ｆm がＭＺＭに印加された時，位相マッチする支配的な縦モードはＰ×ＦＳＲの間隔の周波数である。定常状態の条件のもとで，これはＰ×ＦＳＲの繰り返し周波数をもつ光パルスを生成する。それは，従来のＭＬ−ＦＲＬのものに比べて，複合キャビティをもつＭＬ−ＦＲＬにおいてパルス繰り返し周波数がＰ×Ｋ×ｆm に増加することを意味する。キャビティ内部にＦＦＰフィルタを挿入したことのおもな利点は，Ｐ×ＦＳＲの周波数間隔をもつ支配的な縦モードがフィルタの各Ｐ番目の伝送ピークで通過し，一方，それ以外の強度の小さいランダムに振動するキャビティモードと中間モードはブロックされ，そのために生成された光パルス列の安定性が増大するということである。
【００１２】
図１を参照して，本発明の原理をさらに説明する。本発明は，上記のようにアクティブ・ハーモニカリ・モードロック・ファイバリングレーザ（ＭＬ−ＦＲＬ）において主縦方向モードを適切に選択したことにより安定な光パルス列のパルス繰り返し周波数を増大させるようにしたものであり，ＦＦＰフィルタと複合キャビティの共鳴モードとによりランダムに振動する中間モードを選択的にフィルタリングするものである。
【００１３】
図１（ａ）はキャビティに生成される光パルスの周波数スペクトルを示し，キャビティ共振モードである基本共振周波数ｆc の整数倍の高調波（スーパーハーモニッグモード）のスペクトルと変調器に印加された変調周波数ｆm によりエンハンスメントされた周波数ｆm の間隔の縦モードの周波数スペクトルを示す。
【００１４】
図１（ｂ）はＦＦＰフィルタの伝送特性を示し，この特性によりＦＳＲ＝Ｋ×ｆm の信号は通過し，他の信号は除去される。
【００１５】
図１（ｃ）はＦＦＰフィルタにさらに複合キャビティをファイバリングに挿入した場合の特性を示し，Ｐ×ＦＳＲの周波数スペクトルが逓倍されることを示している。
【００１６】
図１（ｄ）は，パルス繰り返し周波数ＰＲＦ＝Ｐ×ＦＳＲ＝Ｐ×Ｋ×ｆm の繰り返し周波数で出力される光れパルス列を示す。
【００１７】
通常のアクティブ・ハーモニカリ・モードロック・ファイバリングレーザは図１（ａ）に示すように，レーザキャビティにランダムに振動するｆc の整数倍の周波数の振動のうち周波数間隔ｆm の周波数の位相マッチした縦モードの振動をエンハンスメントする。この場合，ＦＳＲ＝Ｋｆm のＦＦＰフィルタが，ＭＬ−ＦＲＬに挿入されると，図１（ｂ）に示すＦＦＰの特性によりｆm の周波数間隔をもつメイン縦モードは，ＦＦＰフィルタの伝送ピークに一致し，そしてフィルタを通過するが，一方，共鳴キャビティモードと中間縦モードはブロックされ，トレース２に示されるようにＫｆm に等しい繰り返し周波数の振動がエンハンスメントされる。さらに，複合キャビティを含む場合，Ｐ×ＦＳＲ（Ｐは整数）のさらに高い周波数の間隔のモードの振動を生成することが可能であり，そのエンハンスメントされた光スペクトルがトレース３（図１（ｃ））に矢印で示されている。これは，ＦＦＰフィルタのＰ番目の伝送ピークに一致する高次の縦モードはフィルタを通過するが，他方の強度の小さい中間モードは阻止される。そのため，Ｐ×ＦＳＲもしくはＰ×Ｋ×ｆm をもつ周波数の光パルスがエンハンスメントされ，Ｐ×Ｋ×ｆm の繰り返し周波数をもつ光パルス列だけを出力させることができる。これは，通常のＭＬ−ＦＲＬに比較してＰ×Ｋｆm だけパルス繰り返し周波数が逓倍されたことであり，図１（ｄ）はこのように出力される光パルス列を示す。
【００１８】
【発明の実施の形態】
図２は本発明の実施の形態１であり，光フィルタと複合キャビティを導入したアクティブ・ハーモニック・モードロック・ファイバレーザの構成を示す。図２（ａ）において，図１１と共通の参照番号は共通部分を示す。Ａは光増幅器であり，増幅特性を有する光ファイバと励起光源により構成されるものである。光増幅器は半導体光増幅器と電源により構成されるものであっても良い（図１０参照）。図２の構成において，利得媒体は，光ファイバにエルビウム（Ｅｒ），ツリウム（Ｔｍ），ネオジウム（Ｎｄ），プラセオジム（Ｐｒ）等の希土類元素をドープすることにより光増幅性能をもつようにしたものである。あるいはフロライド系の光ファイバでも良い。あるいは光増幅器としては，励起光源と光ファイバのラマン増幅を利用することもできる。２５はファブリ−ペロフィルタ（ＦＦＰフィルタ）である。２６は複合キャビティである。２７は光パルス列の繰り返し周波数逓倍部であって，ＦＦＰフィルタ２５と複合キャビティ２６を備えるものである。２８は光遅延線である。
【００１９】
以下の説明では，エルビウムドープ光ファイバ（ＥＤＦ）もしくはＥｒ／Ｙｂ共ドープ光ファイバを使用した場合を例として説明する。１は利得媒体であり，エルビウムドープ光ファイバ（ＥＤＦ）である。なお，光ファイバおよび複合キャビティ２６の各ファイバはいずれもシングルモード光ファイバである。
【００２０】
図２のレーザ装置において，利得媒体１は，１０６４ｎｍのポンプ光をＷＤＭ３で分岐して励起される。３．４８ＧＨｚのＦＳＲをもつＦＦＰフィルタ，１０ＧＨｚのマッハ・ツェンタ強度変調器４および２．５ｎｍの３ｄＢバンド幅をもつ光バンドパスフィルタ１０がレーザキャビティに挿入される。光遅延線２８がキャビティ長のわずかの調整のために使用される。レーザ出力は１０％の光を出力する光結合器９により得られる。複合キャビティ２６は，２つの５０／５０光カップラー（図示せず）および２０１．９６８ｃｍと２００ｃｍの長さの一組のファイバパッチコードＰ1 とＰ2 により構成される。これは，複合キャビティ２６の２つのキャビティのうち，長いキャビティは長さＬ1 ＝６０１０．１３９１ｃｍで短いキャビティは長さＬ2 ＝６００８．１７１１ｃｍをもつ。それぞれのキャビティの共振周波数ｆ’c1＝３．４１８８ＭＨｚ，そして ｆ’c2＝３．４２ＭＨｚであり，そしてその最も低い共通の高調波の周波数は１０．４４１ＭＨｚであり，それはｆ’c1とｆ’c2の３０５４次および３０５３次の高調波である。従って，複合キャビティ２６をもつＭＬ−ＦＲＬ（以下複合キャビティＭＬ−ＦＲＬと称する）において，支配的なキャビティモードは１０．４４ＧＨｚの周波数であり，それは複合キャビティＭＬ−ＦＲＬの周波数ｆ’composite が１０．４４ＧＨｚであることである。このように複合キャビティＭＬ−ＦＲＬに内部キャビティを設けて光フィルタリングすることにより連続発振動作で繰り返し周波数１０．４４ＧＨｚの光パルス列を得ることができる。
【００２１】
図３はそのようにして得られた光パルス出力のＲＦスペクトルを示す。図３に示されるように，１０．４４ＧＨｚの間隔で高調波が生成されていることが示されている。
【００２２】
実験は次のように行なった。まず，ＦＦＰフィルタ（Ｋ＝１に対する）の３．４８ＧＨｚにほぼ等しく３次高調波が複合キャビティ共振周波数の３次高調波に相当する３．４７７５４５Ｈｚの＋２０ｄＢの変調信号を伝送特性曲線のクワドゥラチュールポイントにバイアスしたＭＺＭ４に印加した。キャビティ長は光遅延線２８により精密に調整し，印加変調周波数の３倍（Ｋ＝１，Ｐ＝３）の１０．４３３ＧＨｚの繰り返し周波数をもつ光パルスを発生させた。生成された光パルス列はサンプリングヘッドＳＤ−２６（１７．４ｐｓの立ち上がり時間）をもつサンプリングオシロスコープ（テクトロニクスＣＳＡ）で観測した。
【００２３】
図４（ａ）はそのようにして得られた観測波形を示す。１０．４３３ＧＨｚパルスは永続的に振幅が等しく，小さい振幅ノイズとタイミングジッタの小さいすぐれた安定性をもつことが示されている。
【００２４】
さらに高い繰り返し周波数の逓倍ファクタＰ×Ｋ＞３を検証するために，ＦＳＲの値とｆ’composite を一定に維持した状態で，印加変調周波数をフィルタのＦＳＲのサブハーモニクスに対応するように減少させた。伝送特性曲線のクワドゥラチュールポイントにＭＺＭをバイアスして，ｆm ＝１．７３８５（Ｋ＝２，Ｐ＝３に対する）とｆm ＝１．１５９（Ｋ＝３，Ｐ＝３に対する）の変調信号を印加した。
【００２５】
図４（ｂ）と図４（ｃ）はそのようにして得られた観測波形をそれぞれ示す。それぞれの観測結果から示されるように，１０．４３３ＧＨｚの一定の繰り返し周波数が連続的に生成された。生成されたパルスは小さい振幅ノイズとタイミングジッターの小さい永続的な安定したモードであることが，図４（ｂ）と図４（ｃ）から明瞭である。このことは，本発明により，従来のＭＬ−ＦＲＬから得られるパルスに対して繰り返し周波数がその６倍と９倍に逓倍した光パルスを安定に得られることを示している。
【００２６】
さらに，本発明の構成により得られる繰り返し周波数が１０．４３３ＧＨｚの光パルス列の光スペクトルを，高解像度の光スペトラムアナライザ（０．０１ｎｍの解像度）により観察した。
【００２７】
図５はこのようにして観測した波形を示す。３．４７７５４５ＧＨｚ，１．７３８５ＧＨｚおよび１．１５９ＧＨｚの変調信号を使用して，それぞれ３次，６次および９次の光パルスにより得られた１０．４３３ＧＨｚの光スペクトルを示す。図５からわかるように，縦モードは１０．４３３ＧＨｚのパルス繰り返し周波数にほぼ対応する０．０８３ｎｍ間隔のスペクトルが得られている。測定されたＦＷＨＭ（半値幅）の３次，６次および９次のパルス列の測定されたＦＷＨＭスペクトルバンド幅はそれぞれ０．１９７ｎｍ，０．１６１ｎｍおよび０．１４３ｎｍであった。
【００２８】
図６はＳＨＧオートコリレータにより測定したパルスを示し，３次，６次および９次の光パルス列のそれぞれに対して１５ｐｓ，１７．９０ｐｓおよび２９．８０ｐｓのＦＷＨＭのＳｅｃｈ2 プロファイルに似ていることを示している。それは，それぞれ０．３７，０．３６および０．３７の時間バンド幅積であり，伝送限界に近いことを示している。
【００２９】
さらに，ミリ波スペクトルアナライザと高速度４５ＧＨｚのフォトディテクタを使用し，合成キャビティＭＬ−ＦＲＬで生成される高次光パルス列を観測した。
【００３０】
図７（ａ），（ｂ），（ｃ）はそのように検出された３つの場合のＲＦスペクトルを示す。図７（ａ），（ｂ），（ｃ）に示されるように，ｆm ，２ｆm ，・・・等の印加変調周波数の高調波は，それぞれ３次，６次および９次のパルス列において２７ｄＢ，２６ｄＢおよび２３ｄＢ以上だけ抑制されていることが明らかである。
【００３１】
図８（ａ），（ｂ），（ｃ）は，それぞれ１０．４３３ＧＨｚのＲＦスペクトルを拡大したものであり，各図から示されるように高い解像度をもつバンド幅が観測された。スーパーモードノイズは３次において４０ｄＢ以上，６次において３８ｄＢ以上，そして９次において３２ｄＢ以上，それぞれの光スペクトルにおいて抑制されたことが図８から示される。これらの測定データは，本発明がパルス繰り返し周波数の逓倍を良好に達成できることを示している。
【００３２】
１０．４３３ＧＨｚの光パルス列の振幅ノイズおよび位相ノイスおよびタイミングジッタを，さらにスペクトル領域技術（参考文献１１参照）により観測した。測定されたシングルサイドバンドノイズスペクトルパワー密度分布Ｌ（ｆ）において，３次光パルス列は，１０．４３３ＧＨｚからのオフセット周波数の１０ｋＨｚと１００ｋＨｚにおいてそれぞれ−８８．１７ｄＢｃ／Ｈｚと−９８．４３ｄＢｃ／Ｈｚを示し，一方，１０ｋＨｚと１００ｋＨｚのオフセット周波数において，６次の光パルス列において位相ノイズは−７８．３３ｄＢｃ／Ｈｚと−８４．９５ｄＢｃ／Ｈｚ，および９次の光パルス列に対して位相ノイズは−７３．６７ｄＢｃ／Ｈｚと−８１．０１ｄＢｃ／Ｈｚであった。
【００３３】
図９は１０．４３３ＧＨｚの基本周波数成分で測定された３次，６次および９次のパルス列のＬ（ｆ）分布を示す。さらに，振幅ノイズと位相ノイズの双方を含む全パルスノイズが，１０．４３３ＧＨｚのキャリアから大きいオフセットの周波数領域１００Ｈｚ−１ＭＨｚに渡って積分されたＬ（ｆ）分布から測定された。測定全パルスノイズデータから，０．１７％，０．８％と１．２５％の低い振幅ノイズ，および０．３４ｐｓ，１．０ｐｓおよび１．５ｐｓが，３次，６次および９次のパルス列に対してそれぞれ観測された。通常，タイミングジッタは高次数の高調波での測定位相ノイズから測定され，位相ノイズが支配的である（参考文献１１参照）。しかし，１０．４３３ＧＨｚの高調波の高次数の繰り返し周波数は検出装置の制限バンド幅のために検出できなかった。
【００３４】
アクティブ・ハーモニック・モードロック・ファイバリングにおいて光フィルタと複合キャビティによるパルス繰り返し周波数を逓倍する新規な構成をもつ本発明により，１０．４３３ＧＨｚの繰り返し周波数をもつ３次，６次および９次のスペクトル観測することができることが確認された。また，その低振幅ノイズ，位相ノイズおよびタイミングジッタはいずれも小さくて安定したすぐれた光パルス列であることが確認された。
【００３５】
図１０は本発明の実施の形態２であって光増幅器として半導体光増幅器を使用する場合の構成を示す。図１０において，図２と共通の番号は共通部分を表す。図１０において，Ａは光増幅器である。１７は半導体光増幅器である。１８は電源であって，半導体光増幅器に励起電流を供給するものである。図１０のシステムにおいて半導体光増幅器１７がレーザ光を発生するとともに光増幅する点以外は図５のシステムの動作と同じである。
【００３６】
【発明の効果】
本発明によれば，高次のパルス繰り返し周波数をもつ光パルス列をアクティブ・ハーモニカリ・モードロック・ファイバリングレーザにおいて生成することができる。本発明では，ファイバ ファブリ−ペロフィルタと，わずかに異なる長さの２つの光ファイバで構成される複合キャビティをファイバリングキャビティに挿入する。生成される光パルス列はスペクトル純度の高い安定なものである。そのため，本発明によれば，（ｉ）図２（ａ）に示されるように，電気発振器２１，電気増幅器２２およびマッハ・ツェンダ強度変調器４のような比較的に周波数の低いドライブ電子機器を使用して，非常に高い繰り返し周波数の光パルス列を生成することができる，また，（ii）高い安定性と高い光繰り返し周波の光パルスを必要とする通信システムを簡単な構成で作ることができる。
【００３７】
参考文献の一覧
（１）S. Kawanishi, H. Takara, K. Uchiyama, I. Shake, and K. Mori, "3 Tbit/s (160 Gbit/s x 19 channel) optical TDM and WDM transmission experiment," Electron. Lett., vol. 35, no. 19, pp. 826-827, 1999.
（２）M. Nakazawa, T. Yamamoto, and K.R. Tamura, "1.28 Tbit/s-70 km OTDM transmission using third-and fourth-order simultaneous dispersion compensation with a phase modulator," Electron. Lett., vol. 36, no. 24, pp. 2027-2029, 2000.
（３）Th. Pfeiffer, and G. Veith, "40 GHz pulse generation using a widely tunable all polarisation preserving erbium fibre ring laser," Electron. Lett., vol. 29, no. 21, pp. 1849-1850, 1993.
（４）K.K. Gupta, and D. Novak, "Millimetre-wave repetition-rate optical pulse train generation in harmonically modelocked fibre ring laser," Electron. Lett., vol. 33, pp. 1330-1331, 1997.
（５）K.S. Abedin, M. Hyodo, and N. Onodera, "154 GHz polarization-maintaining dispersion-managed actively modelocked fibre ring laser," Electron. Lett., vol. 36, no. 14, pp. 1185-1186, 2000.
（６）Kamal K. Gupta, Noriaki Onodera, Kazi S. Abedin, and Masaharu Hyodo, "Pulse repetition frequency multiplication via intra-cavity optical filtering in AM mode-locked fibre ring lasers, to be published.
（７）Z. Ahmed, and N. Onodera, "High-repetition rate optical pulse generation by frequency multiplication in actively mode-locked fibre ring lasers," Electron. Lett., vol. 32, pp. 455-457, 1996.
（８）M.Y. Jeon, H.K. Lee, J.T. Ahn, D.S. Lim, H.Y. Kim, K.H. Kim, and E.H. Lee, "External fibre laser based pulse amplitude equalisation scheme for rational harmonic modelocking in a ring-type fibre laser," Electron. Lett., vol 34, pp. 182-184, 1998.
（９）K.K. Gupta, N. Onodera, and M. Hyodo, "Technique to generate equal amplitude, higher-order optical pulses in rational harmonically modelocked fibre ring laser," Electron. Lett., vol. 37, no. 15, pp. 948-950, 2001.
（１０）N. Onodera, "Supermode beat suppression in harmonically mode-locked erbium-doped fibre ring lasers with composite cavity structure," Electron. Lett., vol. 33, no. 11, pp. 962-963, 1997.
（１１）D.Von der Linde, "Characterisation of noise in continuously operating mode-locked laser," Appl. Phys. B, vol 39, pp. 201-217, 1986.
【図面の簡単な説明】
【図１】本発明の原理を説明する図である。
【図２】本発明の実施の形態１の構成を示す図である。
【図３】本発明で出力される光パルスのＲＦスペクトルを示す図である。
【図４】本発明で生成された光パルスをサンプリングオシロスコープで観測した例を示す図である。
【図５】本発明で生成された光パルスのスペクトルをサンプリングオシロスコープで観測した例を示す図である。
【図６】本発明で生成された光パルスをＳＨＧオートコリレータで測定したスペクトルを示す図である。
【図７】本発明で生成された光パルスのスペクトルの例を示す図である。
【図８】本発明で生成された光パルスのスペクトルの例を示す図である。
【図９】本発明で生成された光パルスのシングルサイドバンドスペクトルノイズ分布の測定結果を示す図である。
【図１０】本発明の実施の形態２の構成を示す図である。
【図１１】従来のＭＬ−ＦＲＬの構成を示す図である。
【符号の説明】
Ａ：光増幅器
１：利得媒体
２：光源
３：光結合器
４：変調器
５，６，７：光アイソレータ
８：ポーラライゼーションコントローラ
９：光結合器
１０：光バンドパスフィルタ
１２: 光ファイバ
２１：電気発振器
２２：電気増幅器
２３：光検出器
２４：測定器
２５：ファブリ−ペロフィルタ
２６：複合キャビティ
２７：光パルス繰り返し周波数逓倍部
２８：光遅延線[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical pulse repetition frequency multiplier, and more particularly to an active harmonic mode-locked fiber ring laser (ML-FRL). The optical pulse train generated in the active harmonically mode-locked fiber ring laser (ML-FRL) is multiplied by the repetition frequency and the optical pulse is stabilized.
[0002]
The present invention can be used as an optical pulse generation source for a high-speed optical communication system and an optical time division multiplexing system (OTDM), and stabilizes an optical pulse train having a high repetition frequency required for that purpose. To do. Alternatively, the present invention stabilizes an optical pulse train having a repetition frequency in a microwave frequency band, a millimeter wave frequency band, and higher frequency bands.
[0003]
[Prior art]
Active harmonic mode-locked fiber ring laser (ML-FRL) can generate high repetition frequency optical pulses, so it is a very popular pulse source for OTDM / WDM in high speed optical communication (References 1, 2, etc.). Here, the harmonic RF modulation signal of the cavity resonance frequency is applied to the RF port of the Mach-Zehnder intensity modulator (MZM) placed inside the cavity and biased to the quadrature point of its transmission characteristic curve. The Thereby, an optical pulse having a repetition frequency equal to the frequency of the applied modulation signal is generated. However, in such a laser device, the maximum pulse repetition frequency is limited by the frequency characteristics of the electronic equipment to be driven and the frequency bandwidth of the modulator. For such a high-speed optical communication system, it is important that the repetition frequency of the original optical pulse is high. In order to increase the pulse repetition frequency in ML-FRL, various methods have been proposed, which alleviated the need to prepare a modulator with a large bandwidth or an electronic device capable of generating a high frequency. For example, there is frequency multiplication by placing a Mach-Zehnder intensity modulator (MZM) inside the cavity, and the nonlinear frequency characteristics of the modulator are used to increase the pulse repetition frequency (reference documents 3 and 4). Alternatively, there is a method of optical filtering by placing a fiber Fabry-Perot filter (FFP filter) inside the cavity, and the ML-FRL pulse repetition frequency can be increased by selectively filtering the longitudinal vibration mode with the FFP filter. (References 5, 6 etc.). Another method is to use pf in ML-FRL. m In order to generate an optical pulse train having a repetition frequency of c By detuning by / p, an optical pulse train having a high repetition frequency can be generated (reference documents 7, 8, 9, etc.).
[0004]
FIG. 11A shows the configuration of a conventional active harmonically mode-locked fiber ring laser (ML-FRL). In FIG. 11A, A is an optical amplifier, which is composed of a pumping light source and a gain medium having optical gain performance. Reference numeral 1 denotes a gain medium, which is an erbium-doped fiber (EDF), which is an Er / Yb-doped fiber. Reference numeral 2 denotes an excitation light source. Reference numeral 3 denotes a coupler (optical coupler). Reference numeral 4 denotes a modulator, which is a Mach-Zehnder modulator. Reference numerals 5, 6, and 7 denote optical isolators. Reference numeral 8 denotes a polarization controller (PC). Reference numeral 9 denotes an optical coupler which branches the oscillation light of the optical fiber ring to the photodetector 23 at a ratio of 90:10 (10% of the oscillation light of the optical fiber ring passes through the photodetector 23). . Reference numeral 12 denotes an optical fiber. An electric oscillator 21 generates a high-frequency electric signal. Reference numeral 22 denotes an electric amplifier. Reference numeral 23 denotes a photodetector which converts an optical signal into an electric signal. A measuring instrument 24 measures a signal converted into an electric signal.
[0005]
In FIG. 11A, the laser light generated by the pumping light source 2 pumps an erbium-doped fiber (EDF) that is the gain medium 1, and the resonance frequency f in the optical fiber ring. c And an integer multiple of the laser beam (super harmonic mode) oscillates. The electric oscillator 21 is f c A frequency f that is an integer multiple of m Are applied to a modulator (Mach-Zehnder optical intensity modulator) 4. The modulator 4 has a voltage V b Apply a bias voltage of
[0006]
In the configuration of FIG. 11A, the modulator 4 has a voltage V corresponding to the quadrature bond of the transmission characteristic curve (maximum transmission point in the transmission characteristic curve). b And applied modulation frequency f. m An optical pulse train having a repetition frequency equal to is generated in the optical fiber ring. FIG. 11B shows the repetition frequency f thus generated and output. m The optical pulse train is shown.
[0007]
It is already known that a composite cavity is used as a means for suppressing super mode noise in ML-FRL (reference document 10). As described above, as a method for increasing the pulse repetition frequency in ML-FRL, K × f m There is a method to enhance the resonance mode governed by the frequency interval of the FSR by inserting an FFP filter having a free spatial frequency region (FSR) equal to the ML-FRL cavity and optical filtering in the ML-FRL cavity (K is an integer, f m Is applied modulation frequency). With this method, an optical pulse train with a repetition frequency equal to FSR can be generated by ML-FRL in a continuous oscillation operation. However, in the method using the FFP filter, the maximum pulse repetition frequency is limited to the FSR of the internal cavity FFP filter.
[0008]
[Problems to be solved by the invention]
As described above, in the conventional ML-FRL, the repetition frequency of the generated optical pulse is f. m It was limited to an integer multiple of. Further, the generated light pulse has a problem that the amplitude is unstable.
[0009]
An object of the present invention is to provide an optical pulse repetition frequency multiplying device that can multiply the repetition frequency of an optical pulse obtained by a conventional ML-FRL with a simple configuration and can stabilize the amplitude and the like of the optical pulse. And
[0010]
[Means for Solving the Problems]
The present invention relates to an active harmonic mode-locked fiber ring laser in which an applied modulation frequency f m An integer multiple of the free spatial frequency domain ( F SR = Kf m ) And a composite cavity having a plurality of cavities with different cavity lengths are provided in the main cavity. By repeating the optical filtering inside the cavity by the FFP filter and the harmonics at the resonance frequency of each of the multiple cavities of the composite cavity, the repetition frequency is increased. According to the present invention, the pulse repetition frequency can be multiplied by a simple operation of adjusting the cavity length by the combination of the FFP filter and the composite cavity.
[0011]
The principle of the present invention will be further described in detail. In order to further increase the pulse repetition frequency higher than the FSR of the FFP filter, it is important to enhance the vibration of the higher-order cavity resonance mode having a higher repetition frequency than the FSR. In order to achieve this, in the present invention, the pulse repetition frequency is increased by the internal optical filtering and the composite cavity (see FIG. 2). Here, the composite cavity is composed of a plurality of optical fibers (patch cords) having different lengths and a 50/50 optical coupler. The length of the cavity is the length L of the composite cavity 1 And L 2 A set of patch cords P 1 And P 2 It is adjusted by. L 1 And L 2 (L 1 > L 2 ) Is the cavity resonance frequency f ′ of the long cavity and the short cavity. c1 And f ' c2 Is chosen to interfere at its lowest common frequency so that is equal to a multiple of the FSR. As a result, the composite cavity ML-FRL incorporating the internal FFP is P × FSR or P × f m Enhances the dominant cavity resonance mode with a frequency spacing of. This is because the composite cavity ML-FRL is f ′ composite = P × K × f m A virtual resonance frequency f ′ such as composite It expresses having. Modulation frequency f under active harmonic mode lock operation m Is applied to the MZM, the dominant longitudinal mode that matches the phase is the frequency of the P × FSR interval. Under steady state conditions, this produces an optical pulse with a repetition frequency of P × FSR. Compared to the conventional ML-FRL, the pulse repetition frequency is P × K × f in the ML-FRL having a composite cavity. m Means to increase. The main advantage of inserting the FFP filter inside the cavity is that the dominant longitudinal mode with a frequency interval of PxFSR passes through each Pth transmission peak of the filter, while the other strength is small Randomly oscillating cavity and intermediate modes are blocked, which increases the stability of the generated optical pulse train.
[0012]
The principle of the present invention will be further described with reference to FIG. The present invention increases the pulse repetition frequency of a stable optical pulse train by appropriately selecting the main longitudinal mode in the active harmonically mode-locked fiber ring laser (ML-FRL) as described above. The intermediate mode that randomly vibrates is selectively filtered by the FFP filter and the resonance mode of the composite cavity.
[0013]
FIG. 1A shows a frequency spectrum of an optical pulse generated in the cavity, and the fundamental resonance frequency f which is a cavity resonance mode. c The spectrum of harmonics (super-harmonic mode) that is an integral multiple of and the modulation frequency f applied to the modulator m The frequency f enhanced by m The frequency spectrum of the longitudinal mode at intervals of.
[0014]
FIG. 1B shows the transmission characteristics of the FFP filter, which indicates that FSR = K × f m The other signal passes and other signals are removed.
[0015]
FIG. 1C shows characteristics when a composite cavity is further inserted into the fiber ring in the FFP filter, and shows that the frequency spectrum of P × FSR is multiplied.
[0016]
FIG. 1 (d) shows the pulse repetition frequency PRF = P × FSR = P × K × f. m A light pulse train output at a repetition frequency of
[0017]
As shown in FIG. 1A, a normal active harmonically mode-locked fiber ring laser fluctuates randomly in the laser cavity. c Frequency interval f of vibrations having an integral multiple of m Enhances the vibration of the longitudinal mode with phase matching of In this case, FSR = Kf m When the FFP filter is inserted into the ML-FRL, the FFP characteristic shown in FIG. m The main longitudinal mode with a frequency interval of 一致 coincides with the transmission peak of the FFP filter and passes through the filter, while the resonant cavity mode and the intermediate longitudinal mode are blocked and Kf as shown in trace 2 m A vibration with a repetition frequency equal to is enhanced. Furthermore, if a composite cavity is included, it is possible to generate vibrations in a higher frequency interval mode of P × FSR (P is an integer), and the enhanced optical spectrum is trace 3 (FIG. 1 (c)). ) With arrows. This is because the higher order longitudinal mode, which matches the Pth transmission peak of the FFP filter, passes through the filter, while the other less intense intermediate mode is blocked. Therefore, P × FSR or P × K × f m An optical pulse with a frequency of さ れ is enhanced and P × K × f m Only an optical pulse train having a repetition frequency of can be output. This is P × Kf compared to normal ML-FRL. m The pulse repetition frequency is multiplied only by this, and FIG. 1 (d) shows the optical pulse train output in this way.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 2 shows a configuration of an active harmonic mode-locked fiber laser in which an optical filter and a composite cavity are introduced according to Embodiment 1 of the present invention. In FIG. 2A, the same reference numerals as those in FIG. 11 indicate common parts. A is an optical amplifier, which is composed of an optical fiber having amplification characteristics and a pumping light source. The optical amplifier may be composed of a semiconductor optical amplifier and a power source (see FIG. 10). In the configuration of FIG. 2, the gain medium has optical amplification performance by doping an optical fiber with rare earth elements such as erbium (Er), thulium (Tm), neodymium (Nd), praseodymium (Pr). It is. Or a fluoride type optical fiber may be used. Alternatively, as an optical amplifier, Raman amplification of a pumping light source and an optical fiber can be used. Reference numeral 25 denotes a Fabry-Perot filter (FFP filter). Reference numeral 26 denotes a composite cavity. Reference numeral 27 denotes an optical pulse train repetition frequency multiplying unit which includes an FFP filter 25 and a composite cavity 26. Reference numeral 28 denotes an optical delay line.
[0019]
In the following description, an example in which an erbium-doped optical fiber (EDF) or an Er / Yb co-doped optical fiber is used will be described. Reference numeral 1 denotes a gain medium, which is an erbium-doped optical fiber (EDF). The optical fiber and each fiber of the composite cavity 26 are single mode optical fibers.
[0020]
In the laser apparatus of FIG. 2, the gain medium 1 is excited by branching the 1064 nm pump light by the WDM 3. An FFP filter with a 3.48 GHz FSR, a 10 GHz Mach-Zehnder intensity modulator 4 and an optical bandpass filter 10 with a 3 dB bandwidth of 2.5 nm are inserted into the laser cavity. An optical delay line 28 is used for slight adjustment of the cavity length. The laser output is obtained by an optical coupler 9 that outputs 10% light. The composite cavity 26 consists of two 50/50 optical couplers (not shown) and a set of fiber patch cords P with lengths of 201.968 cm and 200 cm. 1 And P 2 Consists of. This is because the long cavity of the two cavities of the composite cavity 26 has a length L. 1 = 600.1391cm and short cavity is length L 2 = 6008.1711 cm. Resonant frequency f ′ of each cavity c1 = 3.4188MHz and f ' c2 = 3.42 MHz, and its lowest common harmonic frequency is 10.441 MHz, which is f ′ c1 And f ' c2 3054th order and 3053rd order harmonics. Therefore, in ML-FRL having the composite cavity 26 (hereinafter referred to as composite cavity ML-FRL), the dominant cavity mode is a frequency of 10.44 GHz, which is the frequency f ′ of the composite cavity ML-FRL. composite Is 10.44 GHz. Thus, by providing an internal cavity in the composite cavity ML-FRL and performing optical filtering, an optical pulse train having a repetition frequency of 10.44 GHz can be obtained by continuous oscillation operation.
[0021]
FIG. 3 shows the RF spectrum of the optical pulse output thus obtained. As shown in FIG. 3, it is shown that harmonics are generated at intervals of 10.44 GHz.
[0022]
The experiment was performed as follows. First, a modulation signal of +20 dB of 3.477545 Hz, which is approximately equal to 3.48 GHz of the FFP filter (for K = 1) and whose third harmonic corresponds to the third harmonic of the composite cavity resonance frequency, is quadrature of the transmission characteristic curve. Applied to MZM4 biased to point. The cavity length was precisely adjusted by the optical delay line 28 to generate an optical pulse having a repetition frequency of 10.433 GHz, which is three times the applied modulation frequency (K = 1, P = 3). The generated optical pulse train was observed with a sampling oscilloscope (Tektronix CSA) having a sampling head SD-26 (rise time of 17.4 ps).
[0023]
FIG. 4 (a) shows the observed waveform thus obtained. The 10.433 GHz pulse has been shown to have excellent stability with low amplitude noise and timing jitter with permanently equal amplitude.
[0024]
In order to verify a higher repetition frequency multiplication factor P × K> 3, the FSR value and f ′ composite Is maintained constant, the applied modulation frequency is reduced to correspond to the FSR subharmonics of the filter. Bias MZM to the quadrature point of the transmission characteristic curve, f m = 1.7385 (for K = 2, P = 3) and f m = 1.159 (for K = 3, P = 3) modulation signal was applied.
[0025]
FIG. 4B and FIG. 4C show the observed waveforms obtained in this way. As shown from each observation result, a constant repetition frequency of 10.433 GHz was continuously generated. It is clear from FIGS. 4 (b) and 4 (c) that the generated pulse is a persistent stable mode with small amplitude noise and timing jitter. This indicates that the present invention can stably obtain an optical pulse having a repetition frequency multiplied by 6 times and 9 times that of a pulse obtained from the conventional ML-FRL.
[0026]
Furthermore, the optical spectrum of the optical pulse train having a repetition frequency of 10.433 GHz obtained by the configuration of the present invention was observed with a high-resolution optical spectrum analyzer (0.01 nm resolution).
[0027]
FIG. 5 shows the waveform observed in this way. 10 shows the 10.433 GHz optical spectrum obtained with the 3rd, 6th and 9th order optical pulses using the 3.477545 GHz, 1.7385 GHz and 1.159 GHz modulated signals, respectively. As can be seen from FIG. 5, in the longitudinal mode, spectra at intervals of 0.083 nm corresponding to a pulse repetition frequency of 10.433 GHz are obtained. The measured FWHM spectral bandwidths of the third, sixth, and ninth order pulse trains of the measured FWHM (half width) were 0.197 nm, 0.161 nm, and 0.143 nm, respectively.
[0028]
FIG. 6 shows the pulses measured by the SHG autocorrelator, and the 15th, 17.90ps and 29.80ps FWHM sech for the third, sixth and ninth order optical pulse trains, respectively. 2 It is similar to the profile. It has time bandwidth products of 0.37, 0.36, and 0.37, respectively, indicating that it is close to the transmission limit.
[0029]
Furthermore, a high-order optical pulse train generated by the synthetic cavity ML-FRL was observed using a millimeter-wave spectrum analyzer and a high-speed 45 GHz photodetector.
[0030]
FIGS. 7A, 7B, and 7C show the RF spectra in the three cases thus detected. As shown in FIGS. 7 (a), (b), and (c), f m , 2f m It is apparent that harmonics of the applied modulation frequency such as... Are suppressed by 27 dB, 26 dB, and 23 dB or more in the third, sixth, and ninth order pulse trains, respectively.
[0031]
FIGS. 8A, 8B, and 8C are obtained by magnifying the RF spectrum of 10.433 GHz, and a bandwidth with high resolution was observed as shown in each figure. FIG. 8 shows that the super mode noise is suppressed in the respective optical spectra of 40 dB or more in the third order, 38 dB or more in the sixth order, and 32 dB or more in the ninth order. These measurement data indicate that the present invention can successfully achieve multiplication of the pulse repetition frequency.
[0032]
The amplitude noise, phase noise and timing jitter of the 10.433 GHz optical pulse train were further observed by the spectral domain technique (see Reference 11). In the measured single sideband noise spectrum power density distribution L (f), the third-order optical pulse train has −88.17 dBc / Hz and −98.43 dBc / Hz at 10 kHz and 100 kHz offset frequencies from 10.433 GHz, respectively. On the other hand, at the offset frequencies of 10 kHz and 100 kHz, the phase noise is −78.33 dBc / Hz and −84.95 dBc / Hz in the sixth-order optical pulse train, and the phase noise is −73. They were 67 dBc / Hz and −81.01 dBc / Hz.
[0033]
FIG. 9 shows L (f) distributions of third-order, sixth-order, and ninth-order pulse trains measured at a fundamental frequency component of 10.433 GHz. Furthermore, the total pulse noise including both amplitude noise and phase noise was measured from the L (f) distribution integrated over the frequency region 100 Hz-1 MHz with a large offset from the 10.433 GHz carrier. From measured total pulse noise data, low amplitude noise of 0.17%, 0.8% and 1.25%, and 0.34 ps, 1.0 ps and 1.5 ps are third, sixth and ninth order pulse trains. Was observed respectively. Usually, timing jitter is measured from measured phase noise at higher harmonics, and phase noise is dominant (see Reference 11). However, high order repetition frequencies of harmonics of 10.433 GHz could not be detected due to the limited bandwidth of the detector.
[0034]
Third-order, sixth-order, and ninth-order spectrum observations with a repetition frequency of 10.433 GHz according to the present invention having a novel configuration for multiplying the pulse repetition frequency by an optical filter and a composite cavity in an active harmonic mode-locked fiber ring Confirmed that you can. It was also confirmed that the low-amplitude noise, phase noise, and timing jitter were all small and stable, and were excellent optical pulse trains.
[0035]
FIG. 10 shows a configuration when a semiconductor optical amplifier is used as the optical amplifier according to the second embodiment of the present invention. In FIG. 10, the numbers common to those in FIG. 2 represent common parts. In FIG. 10, A is an optical amplifier. Reference numeral 17 denotes a semiconductor optical amplifier. A power source 18 supplies an excitation current to the semiconductor optical amplifier. 10 is the same as the operation of the system of FIG. 5 except that the semiconductor optical amplifier 17 generates laser light and amplifies the laser light.
[0036]
【The invention's effect】
According to the present invention, an optical pulse train having a higher-order pulse repetition frequency can be generated in an active harmonic mode-locked fiber ring laser. In the present invention, a composite cavity composed of a fiber Fabry-Perot filter and two optical fibers having slightly different lengths is inserted into the fiber ring cavity. The generated optical pulse train is stable with high spectral purity. Therefore, according to the present invention, (i) as shown in FIG. 2A, a drive electronic device having a relatively low frequency such as an electric oscillator 21, an electric amplifier 22, and a Mach-Zehnder intensity modulator 4 is provided. Can be used to generate optical pulse trains with very high repetition frequencies, and (ii) communication systems that require high stability and optical pulses with high optical repetition frequencies can be made with a simple configuration. .
[0037]
List of references
(1) S. Kawanishi, H. Takara, K. Uchiyama, I. Shake, and K. Mori, "3 Tbit / s (160 Gbit / sx 19 channel) optical TDM and WDM transmission experiment," Electron. Lett., vol. 35, no. 19, pp. 826-827, 1999.
(2) M. Nakazawa, T. Yamamoto, and KR Tamura, "1.28 Tbit / s-70 km OTDM transmission using third-and fourth-order simultaneous dispersion compensation with a phase modulator," Electron. Lett., Vol. 36, no. 24, pp. 2027-2029, 2000.
(3) Th. Pfeiffer, and G. Veith, "40 GHz pulse generation using a widely tunable all polarisation preserving erbium fiber ring laser," Electron. Lett., Vol. 29, no. 21, pp. 1849-1850, 1993 .
(4) KK Gupta, and D. Novak, "Millimetre-wave repetition-rate optical pulse train generation in harmonically modelocked fiber ring laser," Electron. Lett., Vol. 33, pp. 1330-1331, 1997.
(5) KS Abedin, M. Hyodo, and N. Onodera, "154 GHz polarization-maintaining dispersion-managed actively modelocked fiber ring laser," Electron. Lett., Vol. 36, no. 14, pp. 1185-1186, 2000.
(6) Kamal K. Gupta, Noriaki Onodera, Kazi S. Abedin, and Masaharu Hyodo, "Pulse repetition frequency multiplication via intra-cavity optical filtering in AM mode-locked fiber ring lasers, to be published.
(7) Z. Ahmed, and N. Onodera, "High-repetition rate optical pulse generation by frequency multiplication in actively mode-locked fiber ring lasers," Electron. Lett., Vol. 32, pp. 455-457, 1996.
(8) MY Jeon, HK Lee, JT Ahn, DS Lim, HY Kim, KH Kim, and EH Lee, "External fiber laser based pulse amplitude equalization scheme for rational harmonic modelocking in a ring-type fiber laser," Electron. Lett ., vol 34, pp. 182-184, 1998.
(9) KK Gupta, N. Onodera, and M. Hyodo, "Technique to generate equal amplitude, higher-order optical pulses in rational harmonically modelocked fiber ring laser," Electron. Lett., Vol. 37, no. 15, pp 948-950, 2001.
(10) N. Onodera, "Supermode beat suppression in harmonically mode-locked erbium-doped fiber ring lasers with composite cavity structure," Electron. Lett., Vol. 33, no. 11, pp. 962-963, 1997.
(11) D. Von der Linde, "Characterisation of noise in continuously operating mode-locked laser," Appl. Phys. B, vol 39, pp. 201-217, 1986.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating the principle of the present invention.
FIG. 2 is a diagram showing a configuration of the first exemplary embodiment of the present invention.
FIG. 3 is a diagram showing an RF spectrum of an optical pulse output in the present invention.
FIG. 4 is a diagram showing an example in which an optical pulse generated according to the present invention is observed with a sampling oscilloscope.
FIG. 5 is a diagram showing an example in which a spectrum of an optical pulse generated according to the present invention is observed with a sampling oscilloscope.
FIG. 6 is a diagram showing a spectrum obtained by measuring a light pulse generated by the present invention with an SHG autocorrelator.
FIG. 7 is a diagram showing an example of a spectrum of an optical pulse generated in the present invention.
FIG. 8 is a diagram showing an example of a spectrum of an optical pulse generated in the present invention.
FIG. 9 is a diagram illustrating a measurement result of a single sideband spectral noise distribution of an optical pulse generated according to the present invention.
FIG. 10 is a diagram showing a configuration of a second embodiment of the present invention.
FIG. 11 is a diagram illustrating a configuration of a conventional ML-FRL.
[Explanation of symbols]
A: Optical amplifier
1: Gain medium
2: Light source
3: Optical coupler
4: Modulator
5, 6, 7: Optical isolator
8: Polarization controller
9: Optical coupler
10: Optical bandpass filter
12: Optical fiber
21: Electric oscillator
22: Electric amplifier
23: Photodetector
24: Measuring instrument
25: Fabry-Perot filter
26: Compound cavity
27: Optical pulse repetition frequency multiplier
28: Optical delay line

Claims (7)

光ファイバと光増幅器と光変調をする変調器により構成される光ファイバリングと，高周波電気信号を発生する電気発振器とを備え，該変調器に周波数ｆm の電気信号を印加した時，繰り返し周期がｆm の整数倍となる光パルス列を生成する光パルス繰り返し周波数逓倍装置であって、
該印加周波数ｆm の整数倍の周波数を通過させるフィルタと長さの異なる光ファイバを並列に接続した複合キャビティをそれぞれ該光ファイバリングに備え，
該複合キャビティの長さの長いほうのキャビティで決められる光ファイバリングの共振周波数ｆc 1 と該複合キャビティの長さの短いほうのキャビティで決められる光ファイバリングの共振周波数ｆc 2 との共通高調波の最も低い高調波をｆc ’とし，該フィルタにより決められる通過周波数をＫｆm （Ｋは整数）と表すならば，
ｆc ’はＫｆm の整数倍であり，生成される光パルスの繰り返し周波数がＰ×Ｋ×ｆm （Ｐは整数）であることを特徴とする光パルス繰り返し周波数逓倍装置。When configured optical fiber ring by the modulator to the optical fiber and the optical amplifier and an optical modulator, and an electrical oscillator for generating a high-frequency electrical signals, when applying an electrical signal of frequency f m to the modulator, the repetition period there an optical pulse repetition frequency multiplying device for generating an optical pulse train is an integer multiple of f m,
Filter and length passing the integral multiple of the frequency of the indicia pressurized frequency f m of different optical fiber composite cavity which is connected in parallel with the optical fiber ring,
The common resonance frequency f c 1 of the optical fiber ring determined by the longer cavity of the composite cavity and the resonant frequency f c 2 of the optical fiber ring determined by the shorter cavity of the composite cavity If the lowest harmonic of the harmonic is f c ′ and the pass frequency determined by the filter is represented by Kf m (K is an integer) ,
f c 'is an integer multiple of Kf m, the optical pulse repetition frequency multiplier and wherein the repetition frequency of the generated optical pulse is P × K × f m (P is an integer). 該フィルタはファブリ−ペロフィルタであることを特徴とする請求項１に記載の光パルス繰り返し周波数逓倍装置。2. The optical pulse repetition frequency multiplier according to claim 1, wherein the filter is a Fabry-Perot filter. 該変調器はマッハ・ツェンダ変調器であり，伝送特性曲線のクワドゥラチュールポイントの電圧にバイアスされ，印加する変調信号周波数のサイドバンドを生成することを特徴とする請求項１もしくは２に記載の光パルス繰り返し周波数逓倍装置。3. The modulator according to claim 1, wherein the modulator is a Mach-Zehnder modulator and is biased to a voltage at a quadrature point of a transmission characteristic curve to generate a sideband of a modulation signal frequency to be applied. Optical pulse repetition frequency multiplier. 光増幅器は光増幅特性を有する光ファイバと励起光源により構成されることを特徴とする請求項１，２もしくは３に記載の光パルス繰り返し周波数逓倍装置。  4. The optical pulse repetition frequency multiplier according to claim 1, wherein the optical amplifier is composed of an optical fiber having optical amplification characteristics and a pumping light source. 該光ファイバは稀土類元素をドープしたものであることを特徴とする請求項４に記載の光パルス繰り返し周波数逓倍装置。  5. The optical pulse repetition frequency multiplier according to claim 4, wherein the optical fiber is doped with a rare earth element. 光増幅器はラマン増幅を利用したものであることを特徴とする請求項４に記載の光パルス繰り返し周波数逓倍装置。  5. The optical pulse repetition frequency multiplier according to claim 4, wherein the optical amplifier uses Raman amplification. 光増幅器は半導体レーザ増幅器と励起電流の電源により構成されることを特徴とする請求項１，２もしくは３に記載の光パルス繰り返し周波数逓倍装置。  4. The optical pulse repetition frequency multiplying device according to claim 1, wherein the optical amplifier is constituted by a semiconductor laser amplifier and a power source of an excitation current.
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